The invention relates to a process adapted to provide on a metallic substrate a protective coating based on hexavalent chromium; it also relates, as novel industrial products, to the bath used in the said process and to the commercial form or presentation of the components of the bath.
Metallic substrates which are intended to be protected against corrosion by use of the above-mentioned process, consist especially of metallic parts of small dimensions and of screwed devices for example of steel or cast-iron.
It is already known (see for example the French Patent FR-A- 2,008,770) to fight efficiently, and without subsequent superficial protection, against corrosion of metallic substrates when providing the said substrates with coatings generally obtained using baths comprising
a metal in powder form, especially zinc,
chromic acid in solution,
an organic solvent comprising, in general, one or several alcohols or polyols as for example the tertiobutanol, the dipropyleneglycol or the diethyleneglycol monoethyl-ether,
a wetting agent and
water.
The substrate to be protected is subjected to a treatment comprising:
1. a preparation step, i.e. a degreasing step possibly followed by a mechanical or chemical pickling,
2. a step using, by spraying or by immersion, a bath of the type disclosed hereabove, followed by a dripping or a drying and
3. a cooking step of the substrate thus treated at a generally high temperature, for example of about 300.degree. C.
Proceeding thus, one obtains a coating comprising a single layer; in order to obtain a multilayer coating, the said treatment is repeated as often as necessary.
The abovesaid baths present an important drawback consisting in the fact that they are instable.
As a matter of fact, the strong oxydizing character of the chromic acid reduces the life of the bath, the solvents which are used being oxidized, even at normal temperature, after only some days.
Consequently, the quality of the coating obtained decreases quickly with the age of the bath.
The object of the invention is, above all, to overcome the said drawback and to provide a bath of the type in question whose stability is improved and which consequently can be used during a long duration.